iMac will not recognize SD card

If the internal card reader fails again gve this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

JimW ATX wrote:

Well, after rebooting a couple more times I got the SD slot on the iMac to recognize the card. Sigh... Might be time for a new system. Kinda dreading the transition.

If the internal card reader has problems with cards, but an external one doesn't have problems with the same cards, the solution might be to simply leave an external card reader attached to the Mac, and use it.

If both the internal card reader and an external card reader have problems with particular cards, the suspicion would fall on those cards – that they are damaged, that the flash memory is wearing out, or that there is some problem with their formatting. Then the solution might be to replace the failing cards.

It is not clear if the external card reader is working properly or not..?

It sounds like the internal SD card reader slot is worn out or damaged.

If the external cards reader is working properly, then I would just keep using it instead of the internal reader.

Please make note to self, none of the new silicon 24" iMac's, MacBook Air's or Mac mini's have built-in SDXC card readers. All of those models will all require using a USB-C external card reader.